Remember the Bill de Blasio campaign ad warning that “secretive oil billionaires” had “set their sights on New York City”? It ended with this plea: “Don’t let the Koch brothers buy this election.”

Well, this week the New York Public Interest Group released the top 50 political donors to individual and party accounts in 2013. The Koch brothers didn’t even make the top half of the list.

The No. 1 big spender was hedge-fund maestro James Simons and his wife, who gave $1 million to the New York State Democratic Committee. George Soros, who ranked No. 3, gave $750,000. By contrast, David Koch was way down at 32nd for the $250,000 he gave to the New York State Republican Committee.

Just as interesting, the man who cleaned up here was Gov. Cuomo, a champion of campaign-finance reform. Surely it’s worth noting that if such “reform” passes, the governor will keep the $30 million he’s amassed — and the advantage that gives him over every other candidate.

Yes, this list doesn’t include contributions to Super PACs. But even here the Koch contributions are dwarfed by those of unions and other Democratic interests. Indeed, the day after de Blasio ran his ad attacking the Koch brothers, a pro-de Blasio PAC took out a million-dollar ad buy. That was far more than pro-Lhota Super PACs collected.

So here’s the question for the mayor: Why are the Koch brothers “buying” an election by supporting Republicans while a hedge-fund manager supporting Democrats is not?